I have a long running thread that does house keeping tasks. The failing of the thread is hard to detect given the system will not be immediately affected. I want to make sure this thread never dies. My understand is as long as the code catches everything (Throwable), it will never die. Here is the sample code:
while (true) {
try {
// house keeping logic
} catch (Throwable t) {
// do not do anything
}
}
Is my understanding correct? Is there a better way hosting long running background tasks? I know ScheduledExecutorService can schedule tasks periodically but if the task needs to keep checking some resource, it is probably more efficient to just put everything in a while loop.
You can't have guaranteed long-running thread. However, you can have re-spawning logic if something goes wrong with your thread.
What you can do is have a watcher that will check if the thread is running every x mins.
Please note, the watcher will fail if JVM is shut down or crashed. If you want to re-start the thread even if JVM shuts down, you need to have external monitoring. For this, you can use tools such as supervisord.
public class LongRunningThread extends Thread {
private volatile boolean IS_RUNNING = true;
#Override
public void run() {
while (IS_RUNNING) {
try {
// actions
this.houseKeep();
} catch (Exception e) {
}
}
}
private void houseKeep() {
// housekeeping logic here
}
}
class ThreadWatcher {
private Thread threadToBeWatched;
public ThreadWatcher(Thread threadToBeWatched) {
this.threadToBeWatched = threadToBeWatched;
}
#Scheduled(cron = "*/1 * * * *")
public void checkStatus() {
if (!threadToBeWatched.isAlive()) {
// Logic to restart the thread.
}
}
}
If the housekeeping logic includes some tasks that could be classified as periodic tasks ( e.g check the connectivity to some service once per 20 seconds ) - then you'd be better off with TimerTask
Some example could be of help.
If something (anything short of OOM) happens in the TimerTask - it won't affect the timer that is responsible for task scheduling.
A few points:
I would strongly recommend using ScheduledExecutorService or some other equivalent feature that may be provided by any tools or framework you are using (e.g. #Scheduled in Spring). There are lots of considerations in these situations that are not immediately obvious, and a well established and maintained library will deal with these.
Second, it is uncommon to poll for changes in this manner without some kind of delay. A typical scenario might require you to, for example, close connections with machines that haven't responded in 1 minute. For this sort of loop, you don't need to be constantly checking, as the above loop will do. At the very least, you should include a call to Thread.sleep() to prevent excessive resource use. (This is another issue that ScheduledExecutorService deals with nicely).
Finally, you should include some way to gracefully exit the loop when your application terminates. A simple boolean running = true; on the class would suffice, and you then change your loop to: while (running) {...}. When you want to exit, simply set running = false; and your loop will exit.
I am trying to figure out the solution for a scenario of cancelling a group of threads if one of the Thread finishes the task successfully.
The scenario is like this: Lets say a group of threads are working on a task to find a String in a file and if any of the thread finds the String, the remaining treads should stop execution.
Using 1.5 concurrency we can achieve this, but prior to JDK 1.5 if we want to achieve this kind of scenario, then how can we do that?
For Java 2 Platform, Standard Edition 1.4 SDK or higher you could use exception chaining.
How about just splitting up the thread's task into smaller chunks and checking for a stop condition in the loop?
You could interrupt the threads and in each thread check for the interrupted flag:
public class Job implements Runnable {
#Override
public void run() {
while (!Thread.currentThread().isInterrupted()) {
// perform stuff
}
}
}
In the shutdown-code you call
thread.interrupt();
for each other thread.
The advantage compared to a stop-flag is that your threads will also drop out of any blocking calls.
The good way to stop a thread is to have it periodically check a flag that indicates whether or not it should continue. The java Thread class already has such a facility, called interrupts, that is useful in many scenarios, as bennihepp points out in his answer. See the javadoc for the Thread class and the interrupt() and isInterrupted() methods in particular.
In the following example the runnable class makes sure that all of the instances halt (soon) after one instance finishes. It is worth noting that you should keep your unit of work (one iteration of the loop) small enough.
public class MyRunnable implements Runnable {
private static boolean shouldStop = false;
public void run() {
// allocate required resources
while (!MyRunnable.shouldStop) {
// do work
if (/* some condition to indicate completion */)
MyRunnable.shouldStop = true;
}
// release any allocated resources
}
}
This simple example assumes all threads wrap the same type of a runnable object. Once one sets the shouldStop flag, all others also stop. It would be quite trivial to extend the example to also facilitate interruptions.
I have running thread that I would like to stop and later on resume at some point. I learned not to use stop() etc. as these are deprecated and got to this code below that seems to stop thread successfully. It simply exits run method. Now, how can I restart it? If I call start() method it says that thread is still running, so would directly calling run() do in this situation? Would it cause any problems? BTW this is for Android app if that makes any difference.
private volatile stopThread;
public void stopButton() {
// this is called when button is clicked
// and thread is already started and running
stopThread= true;
}
public void run() {
while (!stopThread) {
// do something here
}
stopThread=false;
}
EDIT: its a timer that starts when thread is started, then can be paused and started again. So timer is a class containing Thread object (I already extend the class with SurfaceView).
The only safe way to stop and resume a thread safely is to add code at the relevant points in the thread's body to deal with it. (Don't use the deprecated Thread stop / pause / resume because they are fundamentally unsafe.)
Stop without resumption is relatively simple, using either an application-specific flag, or the Thread.interrupt() mechanism. The latter is probably better because some of Java's synchronization and IO APIs are interrupt-aware. However, you do run against the problem that a lot of existing libraries are not interrupt aware, or don't deal with InterruptedException properly.
Stop with resumption is more tricky. You'll need to create your own class something like this:
public class PauseControl {
private boolean needToPause;
public synchronized void pausePoint() {
while (needToPause) {
wait();
}
}
public synchronized void pause() {
needToPause = true;
}
public synchronized void unpause() {
needToPause = false;
this.notifyAll();
}
}
and add calls to myPauseControl.pausePoint() at relevant points throughout your the thread's code. Note that this won't allow you to pause IO, or activity in "child" threads, and it will only pause at points in your code where you call the pausePoint method. Also you need to beware of creating problems by pausing the thread while it holds locks on something else, or while something else is waiting for it to respond.
The Java 1.4 docs explained why and gave alternatives to the various deprecated Thread methods, including suspend() and resume() by using wait() and notify() instead.
Look for the heading What should I use instead of Thread.suspend and Thread.resume? about halfway down the page.
The stop() method of Thread class is deprecated and unsafe for use, because stopping a Thread causes it to unlock all monitors that it had locked. This has damaging consequences, because any of the Objects (previously protected by monitors) in an inconsistent state may now be viewed by other threads in an inconsistent state. This behavior may be subtle and difficult to detect.
You never invoke the run() method directly and if you stop the Thread using your volatile variable approach, the Thread would have TERMINATED. In order to start the Thread perform new Thread().start() again.
Say I've got something like this
public void run(){
Thread behaviourThread = new Thread(abstractBehaviours[i]);
behaviourThread.start();
}
And I want to wait until abstractBehaviours[i] run method has either finished or run for 5000 milliseconds. How do I do that? behaviourThread.join(5000) doesn't seem to do that afaik (something is wrong with my code and I've put it down to that).
All the abstract abstractBehaviour class is of course Runnable. I don't want to implement it inside each run method as that seems ugly and there are many different behaviours, I'd much rather have it in the calling/executing thread and do it just once.
Solutions? First time doing something as threaded as this. Thanks!
edit: So the interrupting solution would be ideal (requiring minimal changes to AbstractBehaviour implementations). BUT I need the thread to stop if it has finished OR 5000 milliseconds have passed so something like the following would not work because the thread may finish before the while loop in the parent thread has. Make sense? Any ways around this, I'd love to do it from within the thread that starts the threads obviously.
long startTime = System.currentTimeMillis();
behaviourThread.start();
while(!System.currentTimeMilis - startTime < 5000);
behaviourThread.interrupt();
try {
behaviourThread.join();
} catch (InterruptedException e1) {
e1.printStackTrace();
}
edit: nevermind I see there is a Thread.isAlive() method, all solved I think
The best way to do this is to use the thread interrupt mechanism. The worker thread / Runnable needs to periodically call Thread.interrupted() to see if it is time to stop. The second part of the equation is that a separate thread needs to call Thread.interrupt() on the worker thread after 5000 milliseconds have elapsed.
The advantages of using thread interrupts (over a bespoke solution using flags) include:
The interrupted() state is always available for the current thread. You don't need to pass around an object handle or use a singleton.
An interrupt will unblock some blocking IO and synchronization requests. A bespoke solution cannot do this.
Third-party Java applications and libraries may respect Thread.interrupt().
EDIT - as a commenter points out, you can test whether the current thread has been interrupted using either Thread.interrupted() or Thread.currentThread().isInterrupted(). The main difference between the two approaches is that the former clears the interrupted flag, but the latter doesn't.
You cannot do this externally from the run method - the run method must check some variable to see if it should exit. For example:
class InterruptableRunnable implements Runnable
{
private volatile boolean stop;
public setStop() {
stop = true;
}
public void run() {
while (!stop)
{
//do some work and occassionaly fall through to check that stop is still true
}
}
}
The key thing is for the code in the run loop to check the stop flag occasionally. You can then wire up a timer to set stop to true after 5000 milliseconds.
Finally, it's best practice not to use Threads directly, use the excellent Concurrency Framework. The Concurrency Tutorial is a good place to start and the book Java Concurrency in practice is excellent.
You may do it using java.util.concurrent package.
ExecutorService service = Executors.newCachedThreadPool();
Future future = service.submit(behaviourThread);
long startTime = System.currentTimeMillis();
while (!future.isDone()) {
if (System.currentTimeMillis() - startTime > 5000) {
future.cancel(true);
break;
}
}
// TODO: more work here
//don't forget to shutDown your ThreadPool
service.shutDown();
This code will stop your thread after 5 seconds if it has not finished it's job by that time. If you check behaviourThread.isAlive() it's gonna show false.
You do that by implementing a Runnable
public void run()
{
long time = System.nanoTime(),
end = time + 5 000 000 000; // just better formatting
do {
...my code
} while (System.nanoTime() < end && myOwnCondition);
}
Interrupt is not such a good solution, because you need to access the thread from outside
and it disturbs the program flow. The thread can terminate anytime in your code which
makes cleanup difficult. Please form a habit of letting threads run to the end because otherwise it opens nasty and difficult bugs.
If your program is so heavy duty that you don't know that the while end is reached until the task has completed I suggest the use of a labeled break:
do {
breakout:
{
..my code
if (timetest)
break breakout;
}
// cleanup
...
} while (...);
exampl:
new Thread(new Runnable() {
public void run() {
while(condition) {
*code that must not be interrupted*
*some more code*
}
}
}).start();
SomeOtherThread.start();
YetAntherThread.start();
How can you ensure that code that must not be interrupted won't be interrupted?
You can't - at least not with normal Java, running on a normal, non-real-time operating system. Even if other threads don't interrupt yours, other processes might well do so. Basically you won't be able to guarantee that you get a CPU all to yourself until you're done. If you want this sort of guarantee you should use something like Java Real-Time System. I don't know enough about it to know whether that would definitely provide the facility you want though.
The best thing to do is avoid that requirement in the first place.
Assuming you're only concerned with application-level thread contention, and assuming you are willing to fuss with locks as suggested by others (which, IMHO, is a really bad idea), then you should use a ReadWriteLock and not simple object synchronization:
import java.java.util.concurrent.locks.*;
// create a fair read/write lock
final ReadWriteLock rwLock = new ReentrantReadWriteLock(true);
// the main thread grabs the write lock to exclude other threads
final Lock writeLock = rwLock.writeLock();
// All other threads hold the read lock whenever they do
// *anything* to make sure the writer is exclusive when
// it is running. NOTE: the other threads must also
// occasionally *drop* the lock so the writer has a chance
// to run!
final Lock readLock = rwLock.readLock();
new Thread(new Runnable() {
public void run() {
while(condition) {
writeLock.lock();
try {
*code that must not be interrupted*
} finally {
writeLock.unlock();
}
*some more code*
}
}
}).start();
new SomeOtherThread(readLock).start();
new YetAntherThread(readLock).start();
Actually, you can do this if you control the thread instance you are running on. Obviously, there are a ton of caveats on this (like hanging io operations), but essentially you can subclass Thread and override the interrupt() method. you can then put some sort of boolean in place such that when you flip a flag, interrupt() calls on your thread are either ignored or better yet stored for later.
You really need to leave more info.
You cannot stop other system processes from executing unless you run on a real-time OS. Is that what you mean?
You cannot stop garbage collection, etc unless you run a real-time java. Is that what you wanted?
The only thing left is: If you simply want all YOUR other java threads to not interrupt each other because they all tend to access some resource willy-nilly without control, you are doing it wrong. Design it correctly so that objects/data that NEED to be accessed in a synchronized manner are synchronized then don't worry about other threads interrupting you because your synchronized objects are safe.
Did I miss any possible cases?
Using the synchronized approach ( in the various forms posted here ) doesn't help at all.
That approach only helps to make sure that one thread executes the critical section at a time, but this is not what you want. You need to to prevent the thread from being interrupted.
The read/write lock seems to help, but makes no difference since no other thread is attempting to use the write lock.
It only makes the application a little slower because the JVM has to perform extra validations to execute the synchronized section ( used only by one thread , thus a waste of CPU )
Actually in the way you have it, the thread is not "really" being interrupted. But it seems like it does, because it has to yield CPU time to other threads. The way threads works is; the CPU gives to each thread a chance to run for a little while for very shorts periods of time. Even one when a single thread running, that thread is yielding CPU time with other threads of other applications ( Assuming a single processor machine to keep the discussion simple ).
That's probably the reason it seems to you like the thread is being paused/interrupted from time to time, because the system is letting each thread in the app run for a little while.
So, what can you do?
To increase the perception of no interruptions, one thing you can do is assign a higher priority to your thread and decrease it for the rest.
If all the threads have the same priority one possible schedule of threads 1,2,3 could be like this:
evenly distributed
1,2,3,1,2,3,1,2,3,1,2,3,1,2,3,1,2,3
While setting max for 1, and min for 2,3 it could be like this:
More cpu to thread 1
1,1,1,2,1,1,3,1,1,1,2,1,1,1,3,1,2,1,1,1
For a thread to be interrupted by another thread, it has to be in an interruptable state, achieved by calling, Object.wait, Thread.join, or Thread.sleep
Below some amusing code to experiment.
Code 1: Test how to change the priority of the threads. See the patterns on the ouput.
public class Test {
public static void main( String [] args ) throws InterruptedException {
Thread one = new Thread(){
public void run(){
while ( true ) {
System.out.println("eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee");
}
}
};
Thread two = new Thread(){
public void run(){
while ( true ) {
System.out.println(".............................................");
}
}
};
Thread three = new Thread(){
public void run(){
while ( true ) {
System.out.println("------------------------------------------");
}
}
};
// Try uncommenting this one by one and see the difference.
//one.setPriority( Thread.MAX_PRIORITY );
//two.setPriority( Thread.MIN_PRIORITY );
//three.setPriority( Thread.MIN_PRIORITY );
one.start();
two.start();
three.start();
// The code below makes no difference
// because "one" is not interruptable
Thread.sleep( 10000 ); // This is the "main" thread, letting the others thread run for aprox 10 secs.
one.interrupt(); // Nice try though.
}
}
Code 2. Sample of how can be a thread actually be interrupted ( while sleeping in this case )
public class X{
public static void main( String [] args ) throws InterruptedException {
Thread a = new Thread(){
public void run(){
int i = 1 ;
while ( true ){
if ( i++ % 100 == 0 ) try {
System.out.println("Sleeping...");
Thread.sleep(500);
} catch ( InterruptedException ie ) {
System.out.println( "I was interrpted from my sleep. We all shall die!! " );
System.exit(0);
}
System.out.print("E,");
}
}
};
a.start();
Thread.sleep( 3000 ); // Main thread letting run "a" for 3 secs.
a.interrupt(); // It will succeed only if the thread is in an interruptable state
}
}
Before a thread is interrupted, security manager's checkAccess() method is called.
Implement your own security manager, call System.setSecurityManager to install it and make sure it doesn't let any other thread interrupt you while it is in critical section.
Error processing is an example of a use case where it is very useful to stop threads from being interrupted. Say you have a large multi-threaded server and some external condition arises that causes errors to be detected on multiple worker threads simultaneously. Each worker thread generates a notification that an error occurred. Let's say further the desired response is to bring the server to a safe state that will allow it to restart after the error condition is cleared.
One way to implement this behavior is to have a state machine for the server that processes state changes in total order. Once an error notification arrives, you put it into the state machine and let the state machine process it in toto without interruption. This is where you want to avoid interruptions--you want the first notification to cause the error handler to run. Further notifications should not interrupt or restart it. This sounds easy but really isn't--suppose the state machine was putting the server online. You would want to interrupt that to let error processing run instead. So some things are interruptible but others are not.
If you interrupt the error processing thread it may blow the error handler out of the water during synchronized method processing, leaving objects in a potentially dirty state. This is the crux of the problem--thread interrupts go around the normal synchronization mechanism in Java.
This situation is rare in normal applications. However, when it does arise the result can be byzantine failures that are very difficult to anticipate let alone cure. The answer is to protect such critical sections from interrupts.
Java does not as far as I can tell give you a mechanism to stop a thread from being interrupted. Even if it did, you probably would not want to use it because the interrupt could easily occur in low-level libraries (e.g., TCP/IP socket processing) where the effect of turning off interrupts can be very unpredictable.
Instead, it seems as if the best way to handle this is to design your application in such a way that such interrupts do not occur. I am the author of a small state machine package called Tungsten FSM (https://code.google.com/p/tungsten-fsm). FSM implements a simple finite-state machine that ensures events are processed in total order. I'm currently working on a bug fix that addresses exactly the problem described here. FSM will offer one way to address this problem but there are many others. I suspect most of them involve some sort of state machine and/or event queue.
If you take the approach of preventing interruptions it of course creates another problem if non-interruptible threads become blocked for some reason. At that point you are simply stuck and have to restart the process. It does not seem all that different from a deadlock between Java threads, which is in fact one way non-interruptible threads can become blocked. There's really no free lunch on these types of issues in Java.
I have spent a lot of time looking at problems like this--they are very difficult to diagnose let alone solve. Java does not really handle this kind of concurrency problem very well at all. It would be great to hear about better approaches.
Just start your own sub-thread, and make sure that the interrupt calls never filter through to it.
new Thread(new Runnable() {
public void run() {
Thread t = new Thread() {
public void run() {
*code that must not be interrupted*
}
}
t.start(); //Nothing else holds a reference to t, so nothing call call interrupt() on it, except for your own code inside t, or malicious code that gets a list of every live thread and interrupts it.
while( t.isAlive() ) {
try {
t.join();
} catch( InterruptedException e ) {
//Nope, I'm busy.
}
}
*some more code*
}
}
}).start();
SomeOtherThread.start();
YetAntherThread.start();
I think you need to lock on an interrupt flag. What about something like this (not tested):
new Thread() {
boolean[] allowInterrupts = { true };
#Override
public void run() {
while(condition) {
allowInterrupts[0] = false;
*code that must not be interrupted*
allowInterrupts[0] = true;
*some more code*
}
}
#Override
public void interrupt() {
synchronized (allowInterrupts) {
if (allowInterrupts[0]) {
super.interrupt();
}
}
}
}.start();
SomeOtherThread.start();
YetAntherThread.start();
Best halfway solution would be to synchronize all threads on some common object so that no other threads are runnable while you're in the critical section.
Other than that I do not think it's possible. And I'm quite curious as to what kind of problem that requires this type of solution ?
A usual program does not randomly interrupt threads. So if you start a new Thread and you are not passing the reference to this Thread around, you can be quite sure that nothing will interrupt that Thread.
Keep the reference to the Thread private is sufficient in most scenarios. Everything else would be hacky.
Typically work queues like ExecutorService will interrupt their Thread's when asked to do so. In these cases you want to deal with interrupts.